1. Field of the Invention
This invention relates to multi-phase controlled rectifier systems such as those commonly employed to control the armature voltage and current of a D.C. motor, thereby consequently controlling the torque and speed of the motor.
2. Description of the Prior Art
As is known in the art, rectifier systems are coupled between a multi-phase AC power line and the motor to convert the AC line current to a D.C. current of controllably varied average value. The rectifier systems comprise a plurality of silicon controlled rectifiers (SCR's), half of which are respectively coupled anode-to-cathode from a different one of the power lines to the positive DC output terminal and half of which are respectively coupled cathode-to-anode from a different one of the power lines to the negative DC output terminal. For the purposes herein, the nomenclature adopted to identify the SCR's will indicate the power line and DC terminal coupled thereby. For example, SCR "A+" will be taken to mean the SCR connected between power line A and the positive DC terminal. Additionally, it will be understood that functionally equivalent components could be substituted for SCR's without departing from the scope of the subject invention.
As is also known in the art, the SCR's of a rectifier system are selectively gated in pairs to couple the power lines pair having the greatest voltage between them to the respective output DC terminals. The SCR's are typically gated at controllably varied phase angles in the AC cycle to correspondingly vary the average current coupled into the armature circuit by the SCR's.
One major obstacle to the use of phase control rectifiers has been the uncontrolled phase shifts of power line voltages. These phase shifts are created for example, by the utilities' generators when their load changes; specifically, the phase angle of the generators lag with increasing load. Additionally, utilities will knowingly change the frequency of the line voltage slightly to compensate for accumulated phase changes so that the average frequency from the utility will be constant. Accordingly, the present invention is directed to a timing generator which synchronizes the SCR-gating pulses to the power line phase and thereby tracks such phase changes.